1. Field of the Invention
The present invention relates generally to the construction and operation of solar receivers capable of exposing a flowing stream containing suspended, energy-absorptive particles in the air to focused solar radiation. More particularly, the invention relates to a design of a solar receiver in which the particles are exposed through an open aperture in the receiver.
Power generation using a field of heliostats to focus solar radiation on a central receiver tower has been demonstrated. Current receiver designs expose metal-tubed heat exchangers to the solar radiation and either generate steam directly by heating water in the exchangers or use nitrate salts or liquid sodium as an "intermediate" energy absorbing fluid and transfer the energy to water in order to generate steam for a Rankine-cycle power plant.
Solar receivers to heat air have been proposed which expose ceramic tubes to the solar radiation. However, these systems are unattractive because ot their high cost, large technical risk and low receiver efficiency.
An alternate approach has been suggested for efficiently heating air to a high temperature and using the heated air with a Brayton-cycle gas turbine to generate electricity. Submicron carbon particles suspended in air are exposed to solar radiation from a focused heliostat field. Because of their small size, the particles efficiently absorb the radiation and heat the surrounding air by conduction. The carbon particles eventually become oxidized, leaving a clean, heated airstream. The heated airstream may be used to drive a Brayton-cycle gas turbine which is coupled to an electrical generator.
Compared to solar Rankine-cycle systems, solar Brayton-cycle (gas turbine) systems have the following advantages. They are simple power conversion systems with higher reliability and lower operating and maintenance costs. They can be commercially attractive at smaller sizes. Their inherent low water requirements increase the siting potential for central receivers in arid regions. They are more easily adaptable to fossil hybrid operation which increases the plant availability.
Although the use of solar radiation to heat a suspension of carbon particles promises a significant improvement in the efficiency of solar electrical generation, development of such systems has been hindered because of high cost, fabrication difficulties, and operational limitations associated with the very large windows required for commercial systems. Such windows may be from 5 to 10 meters across, or larger, and must be able to withstand the very high air temperatures which appear to be achievable. Such windows would be very expensive if they are available at all.
Thus, it would be desirable to provide solar receivers which do not require the use of high temperature transparent windows for containing the carbon particle-laden airstream. In particu-ar, it would be desirable to provide solar receivers capable of exposing the particulate-laden airstream through an open aperture.
2. Description of the Background Art
U.S. Pat. No. 4,095,118, to Rathbun describes a self-contained solar power generation system wherein a compressor, particle generator, turbine, and solar receiver are located in a central tower positioned to receive sunlight from a mirror field on the ground. The receiver includes a transparent window to contain the particle-laden working fluid. U.S. Pat. Nos. 4,313,304 and 4,403,601 to Hunt describe a solar receiver having a transparent window and a transparent heat shield. The heat shield is mounted inside the receiver and defines two chambers. By directing clean air througn the chamber between the window and the shield, and particle-laden air through the central chamber, the window is protected from excessively high temperatures. An alternate solar receiver design is described in U.S. Pat. No. 4,499,893 to Hunt et al. A small quartz window at the bottom of the receiver defines a focal plane capable of receiving light from a 360.degree. field. Although minimizing the quartz required for the window, the design further requires a quartz outlet tube to collect heated gases after they have been exposed to solar flux. U.S. Pat. No. 4,452,771 to Hunt and Department of Energy bulletin DOE/TIC/EG-85/046 describes an improved carbon particle generator for use in solar energy systems of the type just described.